For example, a patent document 1 discloses a capacitive humidity sensor in which a detection capacitive device and a reference capacitive device are formed on a common substrate. A capacitance of the detection capacitive device changes with a change in ambient humidity. A ratio of a change in capacitance of the reference capacitive device to a change in the humidity (hereinafter sometimes simply referred to as the ratio) is different from that of the detection capacitive device.
In the patent document 1, the detection capacitive device (a first sensor device) has a pair of detection electrodes (comb-shaped electrodes) and a detection humidity-sensitive film (a humidity-sensitive film). The detection electrodes are located to face each other on the same surface of the substrate, and a detection humidity-sensitive film (a humidity-sensitive film) covers the detection electrodes. A relative permittivity of the detection humidity-sensitive film changes when the detection humidity-sensitive film absorbs water. The reference capacitive device (a second sensor device) has a pair of reference electrodes (comb-shaped electrodes) and a reference humidity-sensitive film (a humidity-sensitive film). The reference electrodes are located to face each other on the surface (the above same surface) where the detection electrodes are located, and the reference humidity-sensitive film covers the reference electrodes. A relative permittivity of the reference humidity-sensitive film changes when the reference humidity-sensitive film absorbs water. According to the patent document 1, the reference humidity-sensitive film and the detection humidity-sensitive film are integrated as a single humidity-sensitive film, and this humidity-sensitive film collectively covers the detection electrodes and the reference electrodes that are located on the same surface of the substrate.
Thus, the reference electrodes are protected by the humidity-sensitive film (a reference humidity-sensitive film). Therefore, compared to a structure in which the reference capacitive device has no humidity-sensitive film, deterioration of the reference capacitive device is reduced so that a variation in the capacitance of the reference capacitive device can be reduced even in hot and humid conditions.